Use new blkback signature based on V1_LWT.BLOCK

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Use new blkback functor

This moves a lot of the boilerplate into the library.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
release 1.2.0

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #12 from djs55/megafunctor

Use new blkback functor